The present invention relates generally to braking systems for motor vehicles which, as is well known, have at least one front axle and at least one rear axle, and each of whose wheels is equipped with a braking device such as a drum brake, a disc brake or a crown brake. The invention relates more particularly to those braking systems which comprise, for each of the said axles, at least one servo-operated control valve which is linked to at least one of the wheels carried by such an axle and which is connected to an anti-locking circuit belonging to the said axle and having a monitoring part and a control part, the electrical supply of the control part being regulated by a safety switch controlled by the monitoring part.
Such a braking system is described, for example, in the French Pat. No. 2,306,856.
In a braking system of this kind, the purpose of the monitoring part of the anti-locking circuit is, for reasons of safety, to effect at specific intervals an examination of all the functions of the anti-locking circuit and to control the operation of the safety switch, which ensures the supply to the control part thereof and thus enables it to act effectively, only if at least some of the said functions are truly capable of being carried out.
In the event of failure in any one of the said functions, the safety switch is left in or is controlled to assume the opening position, thereby rendering the corresponding anti-locking circuit inoperative.
From then on, once the wheels of the axle in question are not subject, during a braking action, to any monitoring by an anti-locking circuit, the wheels can become locked, if braking is excessive, with the attendant risks inherent in such a situation.
In hitherto known systems of this kind, the safety switches connected to the various axles of the vehicle are connected in series.
The result is that, during a braking action, on the assumption of a failure in the anti-locking circuit of the rear axle, the wheels of this rear axle may become locked, whereas, with the anti-locking circuit of the front axle being correctly in operation, the same is not true of the front wheels.
As is known, loss of control at the driving end of a vehicle, which is usually caused by locking of its rear wheels, generally results in a skid which is always dangerous.
It is an object of the present invention to provide a braking system which enables this disadvantage to be avoided.